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The change in diameter will increase your final drive ratio (numerically) by a very small amount, which will increase your RPM's at a given speed by a very small amount. How that effects your mileage depends on your driving habits. My sweet spot is about 65-70 for mileage. If I drive the mountain back roads a lot where speeds are lower than that, then a smaller tire would put me in the sweet spot more often. Which would, theoretically, increase gas mileage. Sounds good on paper anyway. But I would think the difference between those two sizes would be negligible.

A heavier tire usually decreases mileage because it requires more energy to get it moving, and more energy to keep it moving. 16 pounds of rotational weight is a lot. Especially in a 4 cylinder gas engine at high altitude. Maybe not so much in a diesel.

That much rotational weight will also effect braking.

All else being equal, I'd pick the lighter tire.
 

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The change in diameter will increase your final drive ratio (numerically) by a very small amount, which will increase your RPM's at a given speed by a very small amount. How that effects your mileage depends on your driving habits. My sweet spot is about 65-70 for mileage. If I drive the mountain back roads a lot where speeds are lower than that, then a smaller tire would put me in the sweet spot more often. Which would, theoretically, increase gas mileage. Sounds good on paper anyway. But I would think the difference between those two sizes would be negligible.



A heavier tire usually decreases mileage because it requires more energy to get it moving, and more energy to keep it moving. 16 pounds of rotational weight is a lot. Especially in a 4 cylinder gas engine at high altitude. Maybe not so much in a diesel.



That much rotational weight will also effect braking.



All else being equal, I'd pick the lighter tire.


Bigger tire size lowers engine rpm at the equivalent speed.


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Changing the gearing by increasing tire diameter or by other method doesn't affect mileage much because the engine must work harder to compensate. On level ground, the biggest factor by far is speed. I get my best mileage in 4th gear at moderate speeds.
 

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If you're that concerned about gas mileage - you should stay 'stock'.

Or buy a Prius..... >:)

But seriously, you're splitting hairs. Anything you do - whether add more weight, more diameter, more width, lifted suspension, kids in the backseat, groceries in the bed, pissed off at your boss and drive home too aggressively, you're gonna lose gas mileage.

Get what you WANT - for whatever the reason may be. Don't let the gas mileage handcuff you into doing something that you don't really want. And again - if your budget is that tight, you should probably sell the truck.
 

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Discussion Starter · #6 ·
Janster, I appreciate the assumption that I can’t afford the fuel for my diesel or that maybe I should just buy a Prius instead of trying to make rational decisions based on getting information from my fellow forum members. Damn, what was I thinking. It’s almost as if your channeling 08Canyon today.
 

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Tread design and sidewall construction are going to be the biggest factor assuming weight difference is low. But if you go up 10lbs per tire, now weight will matter more than sidewall construction. Tread design is still the biggest factor though. Going from a highway tire to a mudder will really kill your fuel economy although usually that swap will also come with a pretty big weight penalty too so it's a double hit. Hysterisis is the name of the game in fuel efficiency in the world of tires. All those big knobs with wide gaps cause a lot of flex and that flex requires power and power requires fuel.

If you want to maximize fuel economy then stick with the least aggressive tread you can get away with and stick to the lightest load range needed for your towing or off-road plans. I.e. don't buy an off-road specific mudder tire with an E load rating if you only plan on going offroad once or twice a year and only on logging roads. In a situation like that a highway tire like the P rated Michelin LTX will do the job and save you money at the pump as well as be safer while on road. The next step would be a mild all-terrain tire like the Cooper AT3 in a P rating. From there you go to a C or E rated version if you need a more rubust tire for towing or rough offroad conditions. If you need more traction but don't need the more robust casing then you can move to a more agressive tread like the Grappler G2, Duratrac, KO2, etc but keep the P rating to maximize fuel economy. Does this make sense?
 

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Yes, thank you. I’m looking at Cooper At3 in either a 265 70 17 or a 255 75 17. 41 pounds vs 38. Just not sure if the 32 inch tire will cause rubbing issues with just a 2 inch lift.
Good choice! In this instance I think you'll get better fuel economy with the 255/75/17 due to a narrower wind profile, less weight and less gearing (lower cruising RPM). The reduced width will play an even larger role if you plan on removing the air dam.

I cannot confirm rubbing or not. Seems a few people with that size Duratrac have no rubbing issues but I swear I read about soneone with a different tire had a bit of rubbing. FWIW my P265/70/17s rub at full steering lock but only lightly so it's a non issue. My next tire will be a 255/75/17.
 

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Yes, thank you. I’m looking at Cooper At3 in either a 265 70 17 or a 255 75 17. 41 pounds vs 38. Just not sure if the 32 inch tire will cause rubbing issues with just a 2 inch lift.


I have 255 75 17 duratrac with a 2” rc leveling kit. No rubbing. Tons of guys run 265 70 17 but I have actually read more rub with them and they are a smaller tire. I live in the northeast so I prefer the narrower tire in the snow.




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Janster, I appreciate the assumption that I can’t afford the fuel for my diesel or that maybe I should just buy a Prius instead of trying to make rational decisions based on getting information from my fellow forum members. Damn, what was I thinking. It’s almost as if your channeling 08Canyon today.
Put yourself in OUR shoes....... Again, you’re splitting hairs. You’re asking about upgrades that are going to change / ruin your current gas mileage. Yet - you are asking how to ‘save’ gas mileage. What’s more important to you? Looks / Lift or gas mileage?

Believe me, I get it.... Doing your do-diligence and research - which is great. I commend you for that.
 

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The basic factor that affects fuel economy is of course load on the engine from acceleration, pulling something, climbing hills, pushing air aside by driving at higher speeds, etc. Tire size affects it too I suppose, but unless taken to an extreme, tires size seems like a minority variable in the equation.

Have had my diesel for 8,000 miles now, still blown away what it can do fuel economy wise if allowed to just cruise down the road. I am by no means a hypermiler - I enjoy the diesel because I can "get away" with being a bit of a leadfoot (more accurately a "cast aluminum foot") and still average 25.8 mpg lifetime over 8K miles. The stock tire diameter does match up nicely with 6th gear to produce highway efficiency. I do think the diesel's highway economy would not be impacted much by a slightly taller tire. Just set the cruise at 1,800 rpm and if nobody gets in your way, you are gonna break 30 mpg :smile:

My record fuel economy was on a flat to ever so slightly downhill run, probably with a light tail wind, cruise set at 55 mph (heavily patrolled 50 mph road) no traffic. Saw a sustained 70 mpg over a 10 mile stretch. The other outlier was a 35 mile trip, relatively flat freeway, no traffic, cruise set at 60 mph (55 mph road) and saw 42.5 mpg That is just ridiculous. If allowed to tick over at 1,500ish rpm, cruise set on flat ground, the diesel can barely use any fuel and deliver Prius like fuel economy. Who needs a Prius?

I do find I get better over all fuel economy if I use the cruise control when I can, even in moderate hills, which is odd to me. My gas powered vehicles would rev and work hard to maintain speed on hills, the diesel just holds 6th gear and dials up more boost. Purely highway driving at steady and moderate speed, my truck is not that far behind the 4 cylinder Camry that it replaced. Hot rod around town or drive fast and the Camry did not suffer nearly as much as the Canyon does though, which is not surprising. "Hot rodding" being a relative term of course in a Camry or a diesel Canyon :laugh:

The diesel does do steady state light loads really efficiently. Use it that way and it is crazy how far it can go on a gallon of fuel.
 

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Fuel economy is driven by many many factors. Simply trying to equate it to tire mass and diameter isn't going to yield much of an answer for you.

As you go larger, you raise the vehicle which is negative for fuel economy. Also, increased mass is also negative. Although it does improve your lever ratio. By changing the tire size I would venture to guess you wouldn't see much of a difference. Now that assumes that you ONLY change tire size (and associated mass). By doing this, you are likely changing the tire compound along with tread pattern which may change your rolling resistance which could certainly also impact your fuel economy directly. Although, this may also be true just by changing from one tire to another in the same size category.

So, again, the question of fuel economy is far more complex than just tire dia / mass and what you may elect to do, will likely not have a significant impact on fuel economy directly.
 
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